1. Field of the Invention
This invention relates to a video signal recording and reproducing apparatus (hereinafter called a VTR for short) of the kind having rotating heads and more particularly to a rotating head type video signal recording and reproducing apparatus capable of recording in a so-called long-time mode in which a tape-shaped recording medium is allowed to travel at a speed slower than a standard recording speed past heads of narrow width. 2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows an example of allocation of heads in a conventional helical scan type VTR. Heads 1 and 2 serve for recording a signal-processed video signal on a magnetic tape 3 and for reproducing the video signal from the tape. As is known, the video heads 1 and 2 are arranged at different azimuth angles for high density recording. A rotating cylinder 4 carries these video heads 1 and 2. In FIG. 1, an arrow 5 indicates a direction that the tape 3 travels for normal recording and reproduction. Another arrow 6 indicates the direction of rotation of the heads 1 and 2. The rotational speed of the heads 1 and 2 is 1800 r.p.m. where the video signals are NTSC signals.
FIG. 2 is a developement view showing the arrangement of the heads 1 and 2. The heads 1 and 2 which have different azimuth angles 1a and 2a are arranged in different positions with a phase difference of 180.degree. and at the same height in the axial direction of rotation. Such two-head helical scan type VTR's include VTR's of the kind having two different operation modes. One is a long-time mode in which recording can be performed over a long period of time by allowing the magnetic tape 3 to travel at a low speed and the other is a standard mode (or a short-time mode) in which the magnetic tape 3 is allowed to travel at a normal speed. When the heads 1 and 2 are to be used for both the long-time mode and the standard mode in a VTR operable in two modes, the head width of these heads are determined on the basis of the tape speed for the long-time mode. In the long-time mode, the travel speed of the tape is slower than the short-time mode. Accordingly, a recordable track width is narrower in the long-time mode than in the short-time mode. The head width of the heads to be used, therefore, is determined on the basis of the narrower track width.
FIG. 3(A) shows a recording pattern formed on a tape in the long-time mode by the VTR having the head arrangement shown in FIG. 2. In FIG. 3(A), an arrow 9 indicates the travel direction of the head on the tape. Another arrow 10 indicates the travel direction of the tape. A reference numeral 11 denotes a control track on which a control signal (hereinafter called the signal CTL) is to be recorded for the purpose of controlling the travel of the tape during a reproducing operation. A numeral 12 denotes an audio track which is provided for recording an audio signal thereon. Assuming that a recording track 7a has been recorded by the head 1 shown in FIG. 1, another recording track 8a can be considered to have been recorded by the other head 2 which differs from the head 1 in azimuth angle. The signal recorded in each of the tracks 7a and 8a is a video signal covering one field. A video signal for one frame is obtained by adding up the signals recorded in the tracks 7a and 8a. Following these tracks 7a and 8a, other tracks 7b, 8b, 7c, 8c,--are formed one after another without leaving any space between them.
A recording pattern formed on a tape in the short-time mode by the VTR having the head arrangement of FIG. 2 is as shown in FIG. 3(B). Referring to FIG. 3(B), if a track 13a is recorded by the head 1 shown in FIG. 1, a next track 14a is recorded by the head 2. A one field portion of a video signal is recorded in each of the tracks 13a and 14a. One frame portion of the video signal is made up of the signals recorded in these tracks 13a and 14a. Further recording is accomplished in other tracks 13b, 14b, 13c,--,--one after another. Meanwhile, however, parts of the recording medium between these tracks are left unrecorded.
As described above, a conventional VTR capable of recording both a short-time mode and a long-time mode, leaves parts of the tape unrecorded in the short-time mode. The signal-to-noise ratio of such a VTR is inferior to that of VTR's having only a short-time mode (or the standard mode), because the head cannot reproduce the output efficiently.
Also a problem arises during reproduction at a tape speed different from that of a recording operation such as still image reproduction, slow-motion reproduction, or high speed searching reproduction. A large noise bar results from the head scanning each of the unrecorded parts left on a tape which has been recorded in the short-time mode. This degrades the thus reproduced pictures.
FIG. 4(A) shows a scanning operation of the head for still reproduction from a densely written in record (a record leaving no unrecorded space) which is recorded in the long-time mode by the conventional VTR. In this instance, no such a large noise bar occurs. For the still reproduction, the tape is stopped and the head is allowed to trace the record by straddling two tracks 7b and 8b. The tracing operation is accomplished with two heads which are normally used for recording and reproduction and are arranged to have different azimuth angles. Therefore, the record in the track which has been recorded by the head 1 shown in FIG. 1 can be reproduced solely by the head 1. The record in the track which has been recorded by the other head 2 likewise is reproducible only by the head 2. Accordingly, the output wave form becomes that shown in FIG. 4(B). As shown the reproduction output becomes extremely small at the beginning and ending parts of the tracks 7b and 8b. This inevitably degrades the signal-to-noise ratio of the reproduced output. Further, since one field of the video signal being reproduced deviates 1/60 sec. from another field in terms of time, somewhat different video signals would be alternately reproduced if the video signal recorded represents a quick motion. In such a case, the still picture thus obtained would be blurred.